The present invention relates generally to calculating and predicting residual stresses and distortion in cast aluminum components during a quenching or cooling process, and more particularly to rapidly performing such calculating and predicting such that accurate results are obtained without the use of traditional, time-intensive predictive approaches.
With increasing demand to reduce weight and improve fuel efficiency of automobiles, aluminum castings are being more widely used for critical automotive components, such as engine blocks, cylinder heads and suspension parts. Such aluminum castings are often subjected to cyclic loading such that fatigue performance must be taken into consideration when designing such components. These fatigue properties may be significantly and negatively affected by the presence of residual stresses (i.e., those that remain in a component after manufacturing, processing or the like) in general, and in particular by tensile residual stresses in surface layers, including those around fillets, sharp corners, or the like. Such stresses may originate from a variety of sources. For example, macroscopic residual stresses may arise from heat treatment, machining, secondary thermal and mechanical processing and assembling procedures, whereas microstructural residual stresses often result from thermal expansion or contraction mismatch between phases and constituents, as well as from phase transformations.
Aluminum castings often are subjected to a T6/T7 heat treatment to increase their mechanical properties; such treatment generally includes a solution treatment at a relatively high temperature, followed by a quick quench in a cold or cool media (such as water or forced air), then age hardened at an intermediate temperature. Significant residual stresses and distortion may arise, particularly in those castings having complex geometric structures, due to what is typically a high non-uniformity of temperature distribution in the castings during the quenching processes; this non-uniformity is especially pronounced during rapid quenching. In any event, the presence of residual stresses, distortion or the like in aluminum-based castings can significantly and negatively influence a manufactured component's dimensional tolerance and subsequent performance.
There are often determinable levels of residual stresses in manufactured components, and various ways to measure these stresses in such components. Mechanical techniques such as hole drilling, curvature measurements and crack compliance methods are some of the ways of measuring such stresses, as are diffraction techniques, such as electron, X-ray and neutron, as well as magnetic, ultrasonic, piezospectroscopy, photoelasticity and thermoelastic techniques. Mechanical techniques, however, generally are destructive of the component, while the accuracy of diffraction and other non-destructive techniques in measuring residual stresses generally depends on the extent of microstructure variation and geometric complexity of the component structure. In addition, it is generally impracticable to measure residual stresses in every location of a component not only because of the geometric constraints, but also because of the required time and cost to do so.
Computational simulation is one alternate way to predict residual stresses, where analytical or numerical methods can be used in place of the mechanical or non-destructive approaches mentioned above. Finite element analysis (FEA) is one conventional numerical approach, where the large-scale partial differential equations that explain the mechanics of continuous medium can be modeled as an aggregate of discrete points within the medium. One such system that performs residual stress and distortion predictions with a good accuracy can be found in U.S. Pat. No. 8,214,182 entitled METHODS OF PREDICTING RESIDUAL STRESSES AND DISTORTION IN QUENCHED ALUMINUM CASTINGS that is owned by the Assignee of the present invention and herein incorporated by reference.
Depending on the complexity of the component being modeled, FEA-based simulation needs very long computing times (often measured in hours or even days) to ascertain the residual stresses in cast aluminum components that have been subjected to the aforementioned cooling steps. It would be advantageous if such calculating could be done rapidly—specifically in minutes—in order to expedite early design process iteration turnaround of such components, as well as to shorten and reduce the cost of the development cycle of these components (which may include automotive components such as engine blocks, cylinder heads and other aluminum castings that require heat treatment).